1. Technical Field
The invention concerns wireless communications equipment, and more particularly improvements to wireless base stations for cellular communications.
2. Description of the Related Art
Conventional wireless cellular communications systems have a common architecture in which one or more defined cell sites are formed by the placement of one or more base transceiver stations within a geographic area. A cell site is typically depicted as a hexagonal area in which a transceiver is located such that a radio communication link can be established between the cellular system and a plurality of mobile stations within the defined geographic area for the cell. A variety of standards exist for cellular telephone communications. For example, a common cellular system in the United States makes use of the advanced mobile phone service (AMPS). Other common systems include Nordic mobile telephone service (NMT), total access communications service (TACS), global system for mobile communications (GSM), IS-136 TDMA, and IS-95 CDMA systems.
Each of the above-identified systems makes use of a standard architecture associated with the particular system. For example, the basic architecture of a GSM type network is comprised of a base station subsystem (BSS) which includes a base station controller (BSC) and several base transceiver stations (BTS), each of which provides at least one radio cell with one or more radio frequency (RF) channels for communications with mobile subscribers. The purpose of the BSC is to control each of the BTS units within a region. This control process involves several functions, including allocating and selecting RF channels for transmitting each call and controlling handovers of calls from one BTS to another within the BSC's control region. When a mobile subscriber seeks to place a call, the mobile station will attempt to contact a local BTS. Once the mobile station establishes contact with the BTS, the mobile unit and the BTS will be time synchronized for permitting time division multiple access (TDMA) communications. Subsequently, a dedicated bidirectional signaling channel will be assigned to the mobile subscriber by the BSC.
Finally, the BSC will also set up a switching route to connect the mobile subscriber to a mobile switching center (MSC). The MSC provides a communication link from the BSC, and, as a result, all of the BTSs controlled by the BSC, to the public switched telephone network (PSTN), and performs all necessary call and signal routing to other networks to support mobile communications. This arrangement permits a mobile user or subscriber to move from cell to cell and still maintain service. This architecture is also particularly advantageous as it makes possible reuse of carrier frequencies from one cell to another.
The GSM system is designed to work in the 900 MHz and 1800 MHz bands, as well as the 1900 MHz PCS band in North America. GSM is essentially an all digital service. Each RF channel in the GSM system provides eight digital time slots due to the use of TDMA technology. Each of the RF channels are spaced 200 kHz apart from adjacent channels. The eight time slots supported by the TDMA technology enables each RF channel to be shared by more than one user. With TDMA, each user's voice communication is converted to a digital signal, which is allocated among one of the time slots in an assigned RF channel before being transmitted. In a GSM system, TDMA requires that all user subscriber signals using a single RF channel must arrive at the BTS at the proper time. Overlap of signals from the various mobile stations must be avoided and are ensured by proper signal transmission timing.
BTS equipment used in conventional cellular communication systems typically designates specific RF and signal processing equipment for each individual RF channel allocated to the BTS. This designation can most likely be attributed to the fact that each BTS has been conventionally configured to provide communication capability for only a limited number of predetermined channels in the overall frequency spectrum that is available to the service provider. In any case, each BTS is conventionally assigned at its initialization or during its construction, a set of RF channels on which it can communicate with subscribers. These assigned RF channels are generally carefully chosen so that the potential for interference between cells is minimized.
Within a particular BTS, a single omni-directional antenna can be used to receive and transmit signals to all mobile subscribers. However, a more common approach makes use of a plurality of directional antennas at the BTS site to split a cell into separate sectors, effectively transforming the one cell into multiple cells. Dedicated hardware in the BTS units are typically provided for handling communications for each sector. When using a sectorized approach, the RF channels assigned to a particular BTS must be further allocated among each of the sectors, since interference can be caused if multiple sectors processed by the BTS are operated on the same frequency. Each BTS is provided with DSP units to support communications processed by the particular BTS to which the DSP unit has been assigned. Conventional DSP units in such systems are pre-configured to operate on only the particular RF channels which have been assigned to a specific sector of the BTS.
Thus, DSP units are not generally fungible as between sectors of a particular BTS, and therefore these DSP units cannot be allocated from one sector to another. In cell sites that experience heavy traffic, this limitation can result in a poor allocation of system resources.
In particular, one of the problems with using sectorization in wireless base stations concerns trunking efficiency. Normally, a fixed number of RF carriers is assigned to a sector with the BTS concentrating traffic through a common interface to the PSTN. In many instances, traffic needs in one sector can occasionally exceed the sector's RF and processing resources while resources are available in another sector. However, because the number of RF channels allocated to a sector is fixed in conventional BTS system, those resources are blocked and left unused, lowering the trunking efficiency of the base station.
Omni-directional BTSs, i.e., those that are not sectorized, do not suffer from blocking. For example the Erlang capacity of a 2-carrier omni-directional base station is approximately the same as a sectorized base station using 3 carriers (one per sector). In this regard, it is well known that the sectorized system requires more resources. However, omni-directional base stations do not provide as high a degree of coverage as sectorized systems due to lower antenna gain. Another problem with omni-directional systems is that they cannot take advantage of higher frequency reuse schemes, therefore lowering overall system capacity.
Some companies, such as AirNet Communications Corporation of Melbourne, Fla., use a broadband base station (BBS) rather than the BTS described above. Such systems are disclosed in U.S. Pat. Nos. 5,535,240 and 5,940,834. In this BBS, a broadband transceiver is used for transmitting and receiving a single composite wideband RF waveform that is comprised of a number of frequency channels, rather than the multiple narrow-band transceivers used in the BTS for transmitting and receiving individual frequency channels. By replacing the narrow band transceivers of the BTS with a broadband transceiver, this architecture reduces the number of transceivers required to process a given number of frequency channels; however, this alone still does not solve the trunking problem associated with the BTSs. The architecture and configuration of conventional BBSs may still suffer from limited trunking efficiency, as the BBS can still only process a fixed number of calls due to dedicated processing resources serving a specific transceiver and therefore a specific sector.